1. Field of the Invention
The present invention relates to a power output apparatus having an engine, a first motor, and a second motor and also to a method of controlling the same. More specifically, the present invention pertains to a power output apparatus, which can make the output torque of a drive shaft substantially equal to zero, as well as a method of controlling such a power output apparatus.
2. Description of the Related Art
Some proposed power output apparatuses for outputting power generated by an engine transmit the power through mechanical-electrical-mechanical conversion. For example, a power output apparatus disclosed in JAPANESE PATENT LAYING-OPEN GAZETTE No. 53-133814 couples an output shaft of an engine with a rotating shaft of a d.c. motor via an electromagnetic coupling to make the rotating shaft work as a drive shaft. The engine drives one rotor on the side of d.c. field winding of the electromagnetic coupling, while the other rotor on the side of a.c. armature winding drives the rotating shaft of the d.c. motor or the drive shaft. Electric power produced by a slip between the two rotors of the electromagnetic coupling is supplied from the rotor on the side of a.c. armature winding to the d.c. motor via a rectifier. The d.c. motor also receives electric power from a battery to rotate the drive shaft. Unlike conventional fluid-based torque converters, this proposed structure substantially has no energy loss due to the slip. It is accordingly possible to make the energy loss in the power transmission means relatively small by enhancing the efficiencies of the electromagnetic coupling and the d.c. motor.
In the proposed power output apparatus discussed above, however, the drive shaft is driven by the electromagnetic coupling and the d.c. motor, so that a certain magnitude of torque is continuously output from the drive shaft. The proposed structure does not take into account the operations when the output torque of the drive shaft is made approximately equal to zero.